A wood pulping process wherein wood particulates are subjected to a solvent extraction process that removes pitch as well as other wood extractives. The wood particulates are then subjected to a pulping process that is virtually free of volatile organic compound emissions.
As a preliminary matter, wood can be viewed as consisting of two major components, carbohydrates and lignin. Other components constitute a minor part of the wood and manifest as intercellular material, and extraneous substances that are related to the growth of the cells of the tree. The cell walls of the wood are composed of polysaccharides, the chief of which is cellulose. Lignin, on the other hand, is an amorphous substance, partly aromatic in nature, that has been called a xe2x80x9ccementing materialxe2x80x9d or an xe2x80x9cencrusting substance.xe2x80x9d It is insoluble in water and in most common organic solvents. It is also insoluble in acids, but undergoes condensation reactions in the presence of strong mineral acids. Lignin is partly soluble in alkaline solutions and is readily attacked and solubilized by oxidizing agents.
The extraneous substances of wood are deposited as cells grow, or after they reach maturity. Most of these substances are relatively simple compounds, having a low molecular weight. These low molecular weight substances include pectins, proteins, and like substances that are soluble in water or neutral organic solvents. The extraneous substances also include xe2x80x9cwood extractivesxe2x80x9d that include pitch and volatile organic compounds. These naturally-occurring wood extractives are found in both resin canals within the structure of the wood, as well as within the parenchyma cells of the wood.
In general, wood extractives may be divided into a higher molecular weight, higher boiling point fraction, commonly known as xe2x80x9cpitchxe2x80x9d, and a lower molecular weight, lower boiling point fraction that falls within the definition of xe2x80x9cvolatile organic compounds.xe2x80x9d The United States Environmental Protection Agency (EPA) has determined that volatile organic compounds (VOCs) pose an environmental hazard when they are released into the atmosphere. These VOCs are defined in 40 CFR Part 51(s) as xe2x80x9cany compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions.xe2x80x9d Typically these are volatile low molecular weight organic compounds. The EPA has promulgated regulations limiting the quantity of VOCs that a manufacturing facility may release into the atmosphere.
There is no equivalent regulation regarding pitch. However, pitch poses processing and product quality issues in the pulp and paper industry. In pulp mills, the pitch separates from the cellulosic fibers to form a colloid-like suspension that gradually deposits to build up a scale within the process equipment and ducting of the mill. Ultimately, the pulp mill must be shut down so that this pitch scale may be manually removed. In an effort to reduce the frequency of shut-downs to remove pitch scale, pitch scale control chemicals, such as sodium aluminate and alum, are added to the pulping process. While this strategy is partially successful in that it alleviates the equipment fouling problem, it does not eliminate all the problems caused by pitch. Indeed, the addition of scale control chemicals also poses a waste disposal problem since these chemicals are present in the process water. Although this water is recycled, a portion must be treated and disposed of. This, of course, entails additional operating costs for treatment chemicals, labor and facilities.
Colloidal pitch in the process water, as well as some pitch adhering to pulp fibers, cause significant equipment fouling problems in pulp dryers and papermaking machines. In these capital intensive high speed machines, the pulp is formed into continuous webs on high speed fabrics, dewatered, and dried. During these processes, pitch is gradually deposited onto the rolls and machine xe2x80x9cclothingxe2x80x9d of the papermaking machines to form a tacky, gummy surface deposit. This ultimately results in reduced product quality and machine efficiency. Removing the gum can require shutting down the papermaking machine, chemical cleaning or removing the clothing, and cleaning all affected surfaces. This results not only in cleaning costs and paper wastage losses, but also in significant machine downtime with consequent economic loss. Other methods of treatment include the use of continuous cleaning chemicals and equipment. Some of these chemicals may contribute to the release of VOCs and compositions with high biological oxygen demand (BOD) and/or high chemical oxygen demand (COD) into the environment.
Finally, pitch present in the pulp causes a loss of brightness in paper and absorbent products produced from the pulp. To overcome this, the pulp must be bleached, at an added chemical treatment cost.
The release of VOCs into the atmosphere is also a long-standing problem in the pulp and paper industry. To produce boards (oriented strand board, particle, veneer) composite wood products, and paper and pulp products, raw logs or wood fibrous material must be reduced to wood chips, flakes or sawdust. These wood particulates are then further processed, either by bonding together with a suitable glue to make board products, or undergoing pulping and forming processes to produce a variety of papers and absorbent products. However, the processing of logs into wood particulates, and thence into finished products, poses several challenges. Some of these arise from the nature of wood, namely, that it includes not only cellulosic fibers and lignin but also xe2x80x9cwood extractives,xe2x80x9d as discussed above. VOCs occur naturally in timber and the processing of timber into wood particulates facilitates the migration or diffusion of VOCs to chip surfaces from which the compounds vaporize into the surrounding atmosphere. As a practical matter, since the industry requires a large inventory of wood chips for processing into board products and as feedstock in the pulp and paper processes, significant amounts of VOCs are released into the atmosphere from wood chip storage piles. Further, VOCs are also released into the atmosphere during the processing of the wood chips into paper and pulp products.
The distribution of pitch and VOCs from raw wood into the environment and into products may be more easily understood with reference to FIG. 1. As shown, logs 5xe2x80x2 are processed into chips in chip mill 10xe2x80x2 releasing VOCs 2xe2x80x2 to the atmosphere. The chips are stored in mounds 7xe2x80x2 that continue to release VOCs 4xe2x80x2 to the atmosphere. Chips, naturally containing chemical compounds that may produce from about 1 to about 6 wt. % VOCs, for example, are processed in a pulp mill 12xe2x80x2, which can be a mechanical, thermo-mechanical or a chemical pulp mill, to produce cellulosic and fibrous pulps. During this pulping process, cellulosic fibers of the wood are separated from each other thereby allowing entrapped VOCs to diffuse to fiber surfaces and vaporize into the surrounding atmosphere. However, the higher boiling point pitch material remains in the fibers. Treatment chemicals fix a portion of the pitch to the fibers to reduce the rate of mill equipment fouling. The cellulosic pulp produced may be bleached, such as by a chlorine bleaching process, and is then formed into a continuous web and dried on a pulp drier or paper machine 14xe2x80x2. During these processes, a further significant amount of VOCs is released into the atmosphere. The combined chipping, crushing, pulping, and paper or absorbent product making processes release about one-third of the total natural extractives in the wood into the atmosphere (shown by arrows 2xe2x80x2, 4xe2x80x2, 6xe2x80x2, and 8xe2x80x2) as VOCs, and another one-third into effluent water (arrows 20xe2x80x2, 22xe2x80x2 and 24xe2x80x2). The papermill product 15xe2x80x2, such as newsprint, writing paper, or absorbent products, includes the residual of about one-third of the total amount of extractives, mainly pitch with low amounts of VOCs.
The amount of extractives, and VOCs, in wood varies depending upon several factors including wood species, age, and season of felling. However, chips may be expected to contain from 1 to 6 wt. % VOCs. While the percentage of VOCs released into the atmosphere may appear small, relative to wood particulate mass, the actual quantity is nevertheless very significant. For example, a facility may process about 1,000-6,000 tons of wood chips per day. A 6,000 ton per day facility could, therefore, emit as much as 120 tons of VOCs daily. The EPA proposes limiting the amount of VOCs that any wood chip processing facility releases into the atmosphere by regulations requiring permits. Since a wood chip processing facility represents a significant capital investment, operators must take steps to limit VOC emissions while at the same time ensuring that processing equipment operate at or near full capacity for an adequate return on investment. To date, methods for limiting the quantity of VOC emissions have focused on enclosing the atmosphere surrounding any wood chip process that may release VOCs and subjecting air within the enclosure to treatment for the removal of VOCs, before release of the air into the environment. These methods require expensive equipment including large hoods to enclose equipment, fans and ducts for transporting air containing VOCs, condensers for condensing VOCs and incinerators for combusting VOCs. Such equipment not only poses capital cost demands, but also requires operating and maintenance expenses.
The higher boiling portion of the wood extractives, the pitch, presents separate and different problems in processes for treating wood chips to produce paper and pulp products. In a mechanical pulp mill, relying on heat and mechanical stresses to separate wood chips into fibers, a portion of the pitch vaporizes and later condenses to form a pitch scale that includes a gummy, sticky deposit that fouls the pulping equipment. Ultimately, the fouling reaches a point that a shut down of the mill is required so that the pitch scale may be manually and/or chemically removed. Similarly, in the chemical pulping mill some of the pitch separates from the cellulosic fibers to form a colloidal-type suspension. Pitch is deposited from this suspension and gradually builds up as a scale within the process equipment and ducting of the mill. Ultimately, the pulp mill must be shut down so that this pitch scale may be manually or chemically removed. To reduce the frequency of shut-downs to remove pitch scale, additives can be added to the pulping process, such as sodium aluminate and alum in the mechanical pulping process, in an effort to prevent pitch deposition onto equipment surfaces. While this reduces the equipment fouling problem, it does not eliminate the problem. The chemical additives also pose a waste disposal problem since these chemicals are present in the process water. Although this water is recycled, a portion must be treated and disposed of. This, of course, entails additional operating costs for treatment chemicals, labor and facilities.
In the mechanical pulping process, cellulosic fibers produced in a mechanical fiberizing step is combined with sufficient water to produce a pumpable slurry (xe2x80x9cstockxe2x80x9d) of fibers that can then be transported to additional processing equipment (screens, cleaners, and bleaching facilities). The stock is then formed into paper or other absorbent products or a useful pulp. However, these fibers, and water used to transport the fibers, contain pitch that was released from the wood chips during the fiberizing process. This pitch causes significant equipment fouling problems in papermaking machines where the stock is formed into a continuous web on high speed fabrics used to dewater the stock. The web is then dried to complete the papermaking process. During these process steps, colloidal pitch carried in the slurry gradually deposits onto the rolls and machine xe2x80x9cclothingxe2x80x9d of the papermaking machine to form a tacky, gummy surface deposit. This deposit ultimately results in reduced pulp or product quality and machine efficiency. Removing the gummy deposit can require shutting down the papermaking machine, chemical cleaning of the clothing, often requiring removing of the clothing, and cleaning all other affected surfaces. This results not only in cleaning costs and paper wastage losses, but also in significant machine down time. Other methods of treatment include the use of continuous cleaning chemicals and equipment. However, some of these chemicals may contribute to the release of VOCs and compositions with high biological oxygen demand (BOD) and/or high chemical oxygen demand (COD) into the environment. Similar fouling problems due to the presence of residual pitch in the white water (recycled water) of chemical pulping plants causes significant equipment fouling during pulp processing.
There exists a need to reduce the pitch content of pulp and paper fiber to allow the production of paper of improved strength and brightness. However, this reduction in pitch content must be achieved without significant reduction in the yield of pulp from wood. Otherwise, economic losses due to the decline in yield may not be offset by gains from improved product quality. Further, there exists a need to reduce the pitch content of pulp in order to reduce or eliminate the formation of tacky, gummy surface deposits on clothing of pulp or papermaking machines that adversely affect machine efficiency. There is also a need to eliminate VOC emissions from papermaking processes into the environment.
Further, there exists a need to reduce or eliminate the release into the environment of volatile organic compounds from mechanical wood pulping operations that convert wood chips, or other wood particulates, into wood pulp for subsequent processing into product such as paper and absorbent consumer products. Further, there also exists a need to reduce or eliminate the down time, and to reduce the chemical costs, of wood pulping facilities and paper and absorbent product making machines that is caused by the fouling of equipment by pitch that occurs naturally in wood.
Additionally, there exists a need to reduce or eliminate the release into the environment of volatile organic compounds, that occur naturally in wood, from chemical wood pulping operations. There also exists a need to reduce or eliminate the downtime of chemical wood pulping facilities caused by fouling of equipment by pitch that occurs naturally in wood.
In a first aspect, the invention provides paper and absorbent products made from wood pulps that have reduced pitch content and a process for producing these products. The invention provides a paper pulp of superior strength properties and optical properties, without loss of yield. Further, the invention includes a process that includes a wood pulping stage and a paper-forming stage that have substantially reduced emissions of naturally-occurring volatile organic compounds from wood. Pulps manufactured in accordance with the invention have a reduced pitch content. These reduced pitch content pulps substantially reduce or eliminate the formation of gummy, tacky deposits on papermaking machines during the processing of the pulps into paper or absorbent products.
According to a first aspect of the invention, wood particulates are contacted with a solvent for pitch and VOCs. The solvent extracts a substantial portion of both the pitch and VOCs from the particulates, and is separated as a xe2x80x9cmiscellaxe2x80x9d from the leached wood particulates. The extraction removes from about 50 to about 100 wt. % of the VOCs present in the raw wood particulates. Further, the process also removes from about 40 to about 80 wt. % of the pitch. The miscella, including solvent, water, VOCs, and pitch, is subjected to a separation process that reclaims solvent for reuse in the extraction process. The leached wood particulates, now having substantially reduced pitch and VOC contents, are then subjected to chemical or mechanical processes for the production of pulp, with significantly reduced emissions of VOCs. The pulps, having a reduced pitch content, are then formed into paper and absorbent products on papermaking machines, without the attendant pitch deposits that occur in prior art.
The first aspect of the invention provides a superior paper product that is formed from a mass of cellulosic fibers and that has a pitch content at least about 40% less than an expected pitch content, based on the naturally-occurring pitch content of its wood of origin. The product has superior burst index, tear index, tensile index, Scott Bond, Sheffield Smoothness, stiffness and stretch. The product is also of higher density and porosity (seconds/100 ml). Finally, the product is more oleophobic (i.e., less attractive to oils), but can be produced to a predictable degree of oleophilicity. This facilitates subsequent chemical treatment to control oleophilicity to a desired level for particular products. Such products of specified oleophilicity are advantageous in certain printing applications, where the inks are oil-based or oleophilic.
The first aspect of the invention solves long-standing problems of VOC emissions and pitch fouling of equipment by removing pitch from the wood particulates before the pulping step. The first aspect of the invention allows the virtual elimination of pitch scale formation in pulp mills, and on pulp and papermaking machines. This results in significant improvements in mill efficiencies and reduced use of pitch treatment chemicals, in pulp processes and process water, that pose a disposal problem. By providing wood-containing (commonly known as xe2x80x9cmechanicalxe2x80x9d) pulps of superior optical properties (i.e., appearance), the first aspect of the invention reduces the demand for chemical bleaches. Additionally, the first aspect of the invention reduces the BOD and COD of process water, alleviating the need for post environmental treatment.
In a second aspect, the invention provides a mechanical process for fiberizing wood particulates, that substantially reduces the emission of volatile organic compounds into the environment while maintaining the yield of pulp. Moreover, pulp produced from the mechanical process has a reduced pitch content. Consequently, pitch fouling of the mechanical pulping process equipment, and other pulp processing, drying, and papermaking process equipment, is substantially reduced or eliminated. As a result of the reduced pitch content of the pulp, the invention also allows the production of a pulp of superior strength, brightness, and visual properties.
In the mechanical pulping process of the invention, the wood particulate feedstock sent to the refiners is pre-extracted to remove a substantial proportion of the volatile organic compounds from the wood particulates. As a result, the invention substantially reduces or eliminates the emission of volatile organic compounds from chip pulping, and subsequent pulp and paper forming and drying processes. The extraction stage may also substantially reduce the amount of pitch in wood particulates, depending upon the solvent selected, thereby reducing or substantially eliminating pitch fouling of equipment in the pulp processing and subsequent papermaking processes. This invention also significantly reduces chemical costs, such as defoamer, alum, sodium aluminate and caustic costs, required to deal with pitch deposited during the pulping and bleaching operations. Further, as a result of pitch removal, the process of the invention allows the production of a mechanical pulp of superior strength and optical properties.
In the extraction stage, according to the second aspect of the invention, wood particulates are contacted with a solvent for dissolving VOCs and pitch. The extraction step of the invention removes from about 50 to about 100 wt. % of the VOCs present in the raw wood particulates. Further, the extraction step also removes from about 40 to about 80 wt. of the pitch. The solvent extracts a substantial proportion of the VOCs and pitch from the particulates, and is separated as a xe2x80x9cmiscellaxe2x80x9d from the leached wood particulates. The miscella, including solvent, water, VOCs, and pitch, is subjected to a separation process that recovers the solvent for reuse, a pitch product that may be sold as a chemical feedstock or used as a fuel, and a VOC product. The leached wood particulates, now having substantially reduced VOC and pitch contents, are then subjected to a mechanical pulping process for the production of pulp for use in making paper and absorbent products, with significantly reduced emissions of VOCs.
In the practice of the second aspect of the invention, the solvent extracted wood particulates are charged to a mechanical pulp mill where the particulates are subjected to mechanical stresses (that also generate heat) to separate the individual wood fibers of the particulates from each other to produce a fibrous, cellulosic product. During this process VOC emissions are significantly reduced as compared to prior art processes. When combined with sufficient water, the fibrous product forms a pulp that is pumpable. The mechanical pulping process of the invention produces a pulp that has a reduced pitch content, and superior brightness and strength, as a result.
The second aspect of the invention solves a long-standing environmental problem by virtually eliminating the release of VOCs into the atmosphere in mechanical pulp processes. The invention also allows the virtual elimination of pitch scale formation in pulp mills, and on papermaking machines. By removing pitch from the wood particulates before processing, the invention permits the realization of significant cost savings in pulp mills and subsequent papermaking machine operations. Among these benefits are improvements in mill efficiencies.
In a third aspect, the invention provides a chemical wood pulping process of reduced VOC emissions and chemical pulps of reduced pitch content that have superior physical properties. In accordance with the third aspect of the invention, wood particulates are pretreated in a solvent extraction process to remove a significant proportion of the naturally-occurring VOCs and a significant proportion of the naturally-occurring pitch of the particulates. Thus, when the solvent-extracted wood particulates undergo chemical pulping, the process has significantly reduced VOC emissions, and the pulp product has a reduced pitch content. Moreover, due to the reduced pitch content of the wood particulates charged to the chemical pulping process, pitch fouling of wood pulping equipment, and subsequent pulp processing equipment, is substantially reduced or eliminated.
According to the third aspect of the invention, chemical pulping processes are charged with wood particulates that have been extracted with a solvent for VOCs and pitch. The solvent extracts a significant portion of the VOCs and pitch from the particulates. Usually, the extraction removes from about 50 to about 100 wt. % of the VOCs present in the raw wood particulates, depending upon the solvent and the severity of the extraction. Further, the process also removes from about 40 to about 80 wt. % of the pitch. The leached wood particulates are then subjected to chemical processes for the production of pulp with significantly reduced emissions of VOCs.
The third aspect of the invention solves a long-standing environmental problem by reducing or virtually eliminating wood particulate release of VOCs into the atmosphere in chemical pulping operations. Also, by removing all, or a significant proportion of, the naturally-occurring pitch from wood particulates before processing, the invention permits the realization of significant cost savings in pulp mills and papermaking machine operations by virtually eliminating costs associated with pitch fouling. This results in significant improvements in mill efficiencies. This invention also permits reduced use of pitch treatment chemicals, in pulp processes and process water, that pose a disposal problem. Further, the removal of pitch from wood particulates provides brighter wood particulates that resist age-darkening. This allows the production of pulp of higher brightness, thereby reducing the demand for chemical bleaches. Moreover, the pulps of the invention are less oleophilic but can be produced to a predictable oleophilicity. The use of predetermined amounts of a chemical additive can then produce a desired level of oleophilicity in the end productxe2x80x94a useful feature when oil-based inks are used to print on paper produced from the pulp.
Additionally, the BOD and COD of process water are reduced, alleviating the need for post-treatment for environmental purposes. Also, the process reduces the volume of black liquor produced per ton of pulp thereby debottlenecking liquor recovery systems, in particular the recovery boiler, while also allowing energy savings.